Auditory feedback is critical for learning and maintaining spoken language in humans, as evidenced by speech deficits among the hearing impaired and experiments modifying auditory feedback in hearing subjects. However, studies of nonhuman primate vocal systems present a paradox in terms of the importance of auditory feedback in the evolution of vocal control in humans. Research on nonhuman primate infants indicates little change in call morphology during the course of vocal development. In contrast, research on adults suggests that unrelated group members converge on an acoustically similar call type (dialects), suggesting that auditory experience guides vocal production. To address this paradox, I outline a program of research with infant and adult cotton-top tamarins, a species for which we have considerable information concerning vocal production and perception. I propose to modify auditory feedback during call production to determine whether there are short or long-term changes in acoustic morphology, and whether infants and adults differ with respect to such changes, Evidence of acoustically mediated vocal control in tamarins will provide a powerful model system in which to address the neurobiological basis of vocal plasticity, an important step in understanding the neural basis of disorders of language acquisition in humans.